The invention concerns a process for determining the switchover point when producing a die casting or plastic injection moulding.
When producing metal die castings or injection mouldings of plastic or ceramics, plastic material is injected into the cavity of a die or mould by a suitable injection device, after which it is solidified or cured by cooling. The mould is then opened and the moulding ejected. In injection moulding there are two successive phases. During a filling phase, the mould is filled by an injection device usually being controlled according to a filling program so that the delivery rate obeys certain setpoints on a time scale. As soon as the mould is filled, the injection device is usually controlled during a holding pressure phase so that, according to a holding pressure program, the pressure in the cavity reaches certain setpoints, which may likewise be time-dependent.
It is very important that the switchover point, i.e. the instant of changing from the filling program to the holding pressure program, is determined correctly and coincides as exactly as possible with the instant at which the cavity is just filled completely and volumetrically, i.e. without compression. If the switchover is too soon, there will be a danger of the cavity not being completely filled yet, followed by uncontrolled filling under holding pressure. If switchover is too late, excessive pressure will be reached in the cavity, resulting in a moulding that is brittle and liable to fracture due to internal stresses.
It is therefore decisive that the complete filling of the cavity should be detected as exactly as possible before the onset of a drastic pressure rise. One familiar process DE-A-41 40392 attempts to solve this problem by determining approximately the second derivation of the pressure versus time from measured values of the pressure in the cavity at fixed time intervals, deducing that the switchover point has been reached when this value exceeds a fixed threshold and switching over to the holding pressure program.
This solution has, however, proved to be insufficiently reliable, for considerable pressure variations may occur before the mould cavity is completely filled too, causing the second derivative to exceed the threshold briefly and trigger premature switchover. Within the limits of this known process, this shortcoming can be countered only by raising the threshold, which however poses a danger of the complete mould filling being detected too late, so that switchover is delayed.
Furthermore from EP-A-07 07 936, corresponding to U.S. Pat. No. 5,665,283 a process is known whereby the switchover point is determined by the fuzzy logic method. Since in the fuzzy logic system the measured values are processed in successive stages, this approach again gives a switchover point significantly delayed in relation to the instant at which the mould is actually filled volumetrically.
The object of the invention is to further reduce the lag of the indicated switchover point behind the instant of volumetric mould filling, in order to enhance the quality of the mouldings additionally.
This aim is achieved by the invention. At the end of the volumetric filling process, it is known that the mould cavity pressure undergoes an abrupt rise, which is propagated as a compression front throughout the mould cavity within a few milliseconds. Now if the pressures are measured at two points reached by the material and later the compression front at different times, a differential pressure can be calculated from the two measurements. After the abrupt pressure rise mentioned, this differential pressure drops steeply, reflected in an abrupt sign reversal of its gradient. When this drop exceeds an adjustable threshold, this is interpreted as reaching the switchover point, whereby the threshold ensures that chance fluctuations of the pressures and/or their difference are not misinterpreted by the arrangement as reaching the switchover point.
The new process thus reduces the processing of the measured values to a simple differential pressure formation. This takes place very quickly, so that the new process responds much faster to reaching the volumetric filling than that based on fuzzy logic. Moreover the new process demands far less apparatus than the known processes.
In general, a large number of familiar force or pressure measuring elements, such as strain gauges, are suitable for the pressure measurements to be made, but piezoelectric crystals have proved particularly suited. If these have like polarity, the difference is formed in familiar manner from their measured values, possibly after appropriate amplification, whereas with unlike crystal polarity the measured values are added.
The devices for amplifying, subtracting or adding, and the differential pressure falling below the threshold, and for controlling the switchover to the holding pressure phase, are state of the art and available commercially.
The invention will now be explained in more detail with the aid of figures representing a typical embodiment.